When a card terminal (or a card reader) reads a card and transmits payment data to a server, the server approves the payment data and provides the approval result to the card terminal. When a plurality of card terminals reads a plurality of cards simultaneously and transmits payment data to the server simultaneously, the server needs a lot of time to process the payment data and the processing load on the server is high. In addition, when a communication failure occurs between the card terminal and the server, it is difficult for the server or the card reader to promptly receive payment data or approval for the transaction.